Modern communications and radar systems need high performance antennas to cope with electromagnetic interference. These antennas are required to produce narrow beams and low sidelobes, and operate over a wide range of frequencies and scan angles. In addition, these antennas must reduce unwanted signals entering the main beam and/or sidelobes. The increasing problems with electromagnetic interference motivates systems engineers to build antenna arrays with these features.
An antenna array is a group of antenna elements whose output signals are combined to give one output. When the array gets very large, it is often constructed using subarrays, which are groups of antenna elements whose output signals are combined to give one output. In turn, the subarray outputs are added together to yield the array output.
Antenna arrays respond differently to signal depending on signal frequency, power and direction. At a given frequency, the antenna's spatial response is known an antenna pattern. There is generally one main lobe that significantly magnifies the signal. Other smaller lobes, called sidelobes, are spurious. The main lobe is pointed in the direction of signal propagation. Any signals entering the sidelobes are considered interference, noise or jamming. Thus, a lower sidelobe level relative to the main beam is very desirable.
Low sidelobe arrays weight the signals from the antenna elements and the subarray output before adding them together. There are many low sidelobe weighting systems described in the literature. For example see C. A. Balanis, Antenna Theory Analysis and Design, John Wiley and Sons, New York, 1982; and R. E. Collin, Antennas and Radio wave Propagation, McGraw-Hill Book Co., New York, 1985. All the arrays have the same characteristic, namely, the signal from each element receives a different weight. As an example, consider a 70 element array having a -32 dB Taylor low sidelobe amplitude taper. The product of the subarray amplitude weights, b.sub.m, times the corresponding element amplitude weights, a.sub.mn, yields the desired low sidelobe taper. While these amplitude tapers are desirable, they also require a very complex feed network. The complexity arises because all the a.sub.mn and b.sub.m are different. Thus, mass production is not possible, and the array is difficult and expensive to design, build and test.
To make the array antenna more modular is to make the value of a.sub.mn equal to 1 and then amplitude taper only the subarray outputs. Since the subarrays are identical, they can then be mass produced, and mass production of the subarrays provides a significant savings in design and manufacturing costs. However, applying the low sidelobe amplitude weight to only the subarray output b.sub.m produces rather large sidelobes in the far field pattern called grating lobes. These grating lobes are undesirable and defeat the purpose of low sidelobe amplitude taper.